Returning from a deep sleep - what is in a deep sleep?

We read in Job of what has forever vexed us: thoughts in the night.

"In thoughts from the visions of the night, when deep sleep falleth on men..." Job 4:13

from Wikimedia Commons, Schedelsche Weltchronik, 1493.








Curiously, and speaking to our own experience, the author continues that "it stood still, but I could not discern the form thereof..." (Job 4:16)